1. Field of the Invention
The present invention relates to a rotary magnetic head device in a magnetic recording and reproducing apparatus and, more particularly, to a rotary magnetic head device which is capable of controlling the amount of projection of the rotary magnetic head in a magnetic recording and reproducing apparatus such as a VTR apparatus and a DAT apparatus which adopts a helical scanning system.
2. Description of the Related Art
A conventional rotary magnetic head device in a magnetic recording and reproducing apparatus such as a VTR apparatus and a DAT apparatus which adopts a helical scanning system has the structure shown in FIG. 7.
FIG. 7 is a sectional view of a conventional rotary magnetic head device in a magnetic recording and reproducing apparatus. A conventional rotary magnetic head device is composed of a fixed lower drum 1, a rotary shaft 3 which pierces the lower drum 1 at the center thereof and rotatably supported by the lower drum 1 through upper and lower bearings 2, a seat 4 fixed at the upper end of the rotary shaft 1 which projects from the lower drum 1, and an upper drum screwed to the seat 4. A head rest 6 is removably screwed to a predetermined position of the undersurface of the upper drum 5, namely, the surface facing the lower drum 1. To the head rest 6, a magnetic head 7 is fixed in such a manner as to slightly protrude from the outer peripheries of the upper and lower drums 5 at a small gap therebetween. In FIG. 7, the reference numeral 8 represents an upper transformer attached to the seat 4, 9 a lower transformer fixed to the lower drum 1 in such a manner as to be opposed to the upper transformer 8 with a small gap therebetween, 10 and 11 connecting portions, 12 a distribution board and 13 a magnetic tape.
In the conventional rotary magnetic head device having the above-described structure, the upper drum 5 is rotated at a high speed with a constant number of revolutions. The magnetic tape 13 is slightly obliquely wound around the outer peripheral surfaces of the upper and lower drums 5 and 1, and travels at a predetermined speed. The magnetic head 7 comes into contact with the magnetic tape 13 and records or reproduces a video (or sound) signal. The magnetic head 7 is electrically connected to the upper transformer 8 through the connecting portions 10, 11 and the distribution board 12. The upper and lower transformers 8, 9 are magnetically coupled to each other so as to transmit a signal to each other. The lower transformer 9 is connected to an external signal processor (not shown).
With the travel of the magnetic tape 13 and the rotation of the magnetic head 7, the magnetic head 7 consecutively crosses the magnetic tape 13 obliquely. The trajectories of the magnetic head 7 which crosses the magnetic tape 13 are parallel to each other. This will be explained in more detail with reference to FIGS. 8A and 8B. The reference numeral 13a represents a trajectory of the magnetic tape 13, the symbol V.sub.1 the ordinary magnetic tape feeding speed, the reference numeral 7A the trajectory of the magnetic head 7 and the symbol V.sub.0 the rotating speed of the magnetic head 7. The trajectories 13a and 7A cross each other, as shown in FIG. 8A. Thus, the relative trajectory of the magnetic head 7 which actually comes into sliding contact with the travelling magnetic tape 13 is the one indicated by A in FIG. 8A.
Since the two magnetic heads consecutively come into sliding contact with the magnetic tape 13, if the trajectory of one magnetic head which comes into sliding contact with the magnetic tape is represented by A and the trajectory of the other magnetic head which comes into sliding contact with the magnetic tape is represented by B, the trajectories A and B are formed on the magnetic tape 13 in parallel with each other and at a high density, as indicated by A.sub.1, B.sub.1, A.sub.2, B.sub.2 . . . . These A.sub.1, B.sub.1, A.sub.2, B.sub.2 . . . are what is called tracks on the tape. In the magnetic tape 13, at the time of still reproduction (still operation) and recording stop and standby (pause operation), the travel of the magnetic tape 13 is stopped while the upper drum 5, namely, the rotary drum is rotating at a high speed, so that each magnetic head 7 continuously traces the trajectory 7A shown in FIG. 8A. In other words, the magnetic head 7 continues tracing a position of the magnetic tape 13 at an angle smaller than the ordinary angle of inclination. In this way, the rotary magnetic head device of, for example, a conventional magnetic video recording and reproducing apparatus (VTR) carries out an ordinary reproducing operation (the magnetic tape 13 travels at a predetermined feeding speed), a still reproducing operation (the magnetic tape 13 stops) and a recording stopping and standby operation (the magnetic tape 13 stops).
In 1-inch helical VTRs used for broadcasting, a technique of automatically tracking at the time of reproduction while moving the magnetic head in the direction of the width of the magnetic tape such as what is called automatic scan tracking (AST) or dynamic track following (DTF) has conventionally been developed.
This technique will be explained in the following with reference to FIGS. 9 to 12. In FIGS. 9 to 12, the reference numeral 7 represents the magnetic head for recording and reproduction, 14 a head exclusively for special reproduction which is operated at the time of AST or DTF, 15 a bimorph piezoelectric element to the end of which the head 14 exclusively for special reproduction is attached, and 5 the rotary drum which rotates at 1,800 rpm and to which the magnetic heads 7 for recording and reproduction and the bimorph piezoelectric elements 15 are attached
FIG. 10 shows the structure of the actuator portion of the DTF system, wherein the reference numeral 1 represents the fixed drum and 13 the magnetic tape.
FIG. 11 shows the principle of the operation of the bimorph piezoelectric element 15 which is conventionally used. The bimorph piezoelectric element 15 is a bimorph plate composed of two piezoelectric elements X.sub.1, X.sub.2 sandwiched between electrodes Y.sub.1 and Y.sub.2, and between electrodes Y.sub.2 and Y.sub.3, respectively. To the free end of the bimorph piezoelectric element 15 is attached the head 14 exclusively for special reproduction, and a power source S is connected to the three electrodes Y.sub.1 to Y.sub.3. For example, when the power source S applies a forward voltage to the piezoelectric element X.sub.1 and a reverse voltage to the piezoelectric element X.sub.2, the piezoelectric element X.sub.1 contracts in the direction indicated by the arrow x.sub.1 while the piezoelectric element X.sub.2 extends in the direction indicated by the arrow x.sub.2. On the other hand, when the power source S applies a reverse voltage to the piezoelectric element X.sub.1 and a forward voltage to the piezoelectric element X.sub.2, the piezoelectric element X.sub.1 extends while the piezoelectric element X.sub.2 contracts. In this way, the bimorph piezoelectric element 15 vertically moves in the axial direction of the drums 1 and 5, as shown in FIG. 10.
The principle of noiseless high-speed search at the time of high-speed reproduction will now be explained with reference to FIG. 12. The symbols A.sub.1, B.sub.1, A.sub.2, B.sub.2, A.sub.3, B.sub.3, . . . denote the video tracks recorded on the magnetic tape 13. The azimuth at which the tracks A.sub.1, A.sub.2, A.sub.3, . . . are recorded by the head is different from the azimuth at which the tracks B.sub.1, B.sub.2, B.sub.3, . . . are recorded.
It is now assumed that signals are reproduced at a quintuple speed for high-speed search. Unless the bimorph piezoelectric element 15 for the head 14 exclusively for special reproduction is driven, the trajectory of the head 14 exclusively for special reproduction which scans the magnetic tape 13 is the one represented by the broken line L.sub.1 in FIG. 12. If the azimuth of head 14 exclusively for special reproduction is the same as that of the track A, the signal is reproduced only by scanning the tracks A.sub.1, A.sub.2, A.sub.3, . . . , and the tracks B.sub.1, B.sub.2, B.sub.3 which are recorded at a different azimuth must be scanned separately therefrom, so that a noise bar is produced on a reproducing screen. In order to completely reproduce the track A.sub.1 even when the video tape travels at a quintuple speed, the head 14 exclusively for special reproduction is moved in the axial direction of the track (left-hand side in FIG. 12) by 4 track pitches in one field during which the head 14 exclusively for special reproduction comes into contact with the magnetic tape 13. In the next field, the opposite head 14 exclusively for special reproduction is moved by 4 track pitches. In this way, the tracks A.sub.1, B.sub.3, A.sub.6, . . . are completely scanned, thereby obtaining noiseless reproducing signals. In other words, the noiseless reproduction at any speed is realized by moving the rotary magnetic head in the direction of the width of the track in accordance with the tape speed at the time of reproduction, as described above. That is, the movable head in the conventional device explained above is a head which is movable in the axial direction of the drum.
As described above, a movable head has been developed as a new technique for a VTR and has already been used in some VTR's for business use, but the magnetic head provided on the rotary drum in a household VTR is still a fixed head.
In a conventional rotary magnetic head device, during still reproduction and recording stop and standby, since the magnetic tape 13 is stopped, the magnetic head 7 repeatedly slides on one track (more accurately, the magnetic head 7 slides on the track portion bestriding two tracks because the trajectory of the head has a smaller angle of inclination during the stop of the magnetic tape 13 than the ordinary angle of inclination of the track, as described above) of the magnetic tape 13 at a high speed, as shown in FIG. 8A. The magnetic surface of the magnetic tape 13 is damaged thereby and the magnetization of the magnetic material is gradually reduced (this phenomenon is called demagnetization). To prevent this, in a conventional VTR apparatus, when still reproduction or recording stop and standby continues for, generally, 4 to 5 minutes, the loading of the magnetic tape on the drum is automatically released and the VTR apparatus is stopped. Once the stopping mode is taken, several seconds is required for starting the next operation. In this way, handling of the VTR apparatus during still reproduction or recording stop and standby is very troublesome.
In a VTR of an AST or DTF system, a movable head is adopted as a magnetic head and the vertical movement of the magnetic head in the axial direction of the drum is controlled.
In either case, the magnetic head slightly projects from the outer periphery of the drum and slides on the magnetic surface of the magnetic tape which is wound around the drum, as shown in FIG. 9. Such a magnetic head wears by, for example, 8 to 10 .mu.m in 1,000 hours. Although it is desirable that the amount of projection from the outer periphery of the drum is the same, the amount of projection is apt to become nonuniform due to the wear caused by the sliding on the magnetic tape, and a jitter is disadvantageously produced on the reproduced picture. When the amount of projection from the drum is reduced due to the wear of the magnetic head, the state in which the magnetic head is in contact with the magnetic tape is deteriorated, thereby making it impossible to maintain good picture quality for a long time and, in the worst case, making it necessary to replace the head in spite of short-time use.